On bars these days you find more and more usage of scales and thermometers but less and less interest in calibration. Like most things, there is a balance between dependence on something and willingness to check that what we depend on is accurate.
Here are a few things to calibrate and a few easy ways to calibrate them.
Scales: Some scales come with a gram weight which can be used for calibration. Alternatively, you can purchase a set of weights for calibration purposes. For your small scale that would be measuring amounts such as gram doses, a nickel is a good calibration tool. A nickel weighs 5g and once you have zeroed and tared your scale, you can simply place a nickel on it to see if it is accurate. A better way of calibrating a scale though is to measure the maximum capacity of the scale. If the scale maxes out at 5lb, finding something like a 5lb bag of sugar to measure the maximum of the scale. RTFM applies in all situations as many models of scales have calibration modes outlined in the manual.
It is also a good idea to make sure the scale you use for measuring small gram dose weights has a small enough resolution to be practical. Some scales have resolutions that vary as much as 5g as they are designed for weighing larger amounts, some are used for precision measurements of small amounts but need to be tared often or reset and tared between measurements.
Thermometers: Thermometers are easy to calibrate and hopefully everyone manual brewing coffee is measuring temperature. To calibrate a thermometer, simply fill a cup with mostly ice and a little bit of water allowing more than 80% of the probe to be submerged. Let the thermometer sit submerged for one minute and then observe the temperature. It should measure at 32f/0C. In conjunction, a measure of water at a rolling boil can give another data point. Placing the same probe into water at a rolling boil should give you a measuring point of 212f/100C unless you are at high elevation to which your boiling point may differ. (To verify espresso temperatures, we recommend getting a Scace and calibrating it the same way in both ice water and boiling to establish group head temperatures.)
Note: The usefulness of calibrating thermometers was most apparent for us. With our installation of two roasters (identical), we found the temperature probes (and spares) had a range of variance within 2 degrees. After identifying the offsets, we then programmed this into the displayed temperatures. When changing the temp probes, we would have to repeat the process of calibration.
Flow Meters: The traditional flow meter in an espresso machine is one of the most difficult items a professional barista can use. In many cases, we tend to abandon this because constant recalibration is needed. The reason being that in the dual boiler machines we use, the volumetric component isn't designed for precision and is placed in a position where hot water sits and scale buildup occurs. This creates many problems that makes it less than useful and often a hinderance. Recalibrating involves either a scale weight of the resulting shot (remember mL and grams are exchangeable) or a volumetric estimate with a 'shot glass.'
In the next year, a new type of flow meter will be introduced in coffee brewing that should not be confused with the volumetric controls on an espresso machine. The LuminaireCoffee volumetric controls are a generation ahead in that they function with more accuracy and are more useful in what they display. As opposed to the volumetric controls on an espresso machine, the LuminaireCoffee flow meter is in the cold water section and has less issues with buildup or hard water. It also is precision and digital which is best exemplified when using it in manual mode as it functions similar to a chronos display of time on a La Marzocco espresso machine. It simply displays total cumulative volume and time counts (and a dynamic flow in either ml/s or oz/min) as you are brewing. (In preset mode, it has cutoffs that can be set to stop the flow at either time or volume points.) Calibration of this is done at production but can also be done by measuring a specific volume of water in a specific time count with a calibrated scale. It makes brewing onto a scale to measure volume a bit superfluous (once calibration is verified). See Flow and Profiling posts for more relevant discussion.